Emergency Medical Technician (EMT) Intermediate Practice Exam

What causes normal inspiration in the human body?

1. Positive pressure in the thoracic cavity

2. Negative pressure in the thoracic cavity

3. Contraction of the abdominal muscles

4. Relaxation of the diaphragm

The correct answer is: Negative pressure in the thoracic cavity

Normal inspiration in the human body is primarily caused by negative pressure in the thoracic cavity. During inhalation, the diaphragm contracts and moves downward, while the intercostal muscles between the ribs also contract, expanding the thoracic cavity. As the volume of the thoracic cavity increases, the pressure inside the cavity decreases relative to the atmospheric pressure outside the body. This negative pressure creates a gradient that allows air to flow into the lungs. When the thoracic cavity's pressure drops below atmospheric pressure, air is drawn in through the nose or mouth and travels into the lungs, filling them with oxygen-rich air. This process is essential for normal respiratory function, allowing for gas exchange in the alveoli of the lungs. Understanding this dynamic is crucial, as it highlights the mechanics of breathing and the importance of pressure gradients in respiratory physiology. The other options suggest mechanisms that do not accurately describe this fundamental process.